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A tailocin is a relatively new term used to describe a bacteriocin that resembles a phage particle consisting of the tail and tail fibers, but missing its head and genomic material (Figure-

1). The tailocin is functionally able to attach to the host bacterial cell and depolarize the plasma

membrane to kill the bacterium (Ghequire and DeMot, 2015). Tailocins appear to kill bacteria by the same hole-forming process that is used for injection of the phage genomic material. In nature, tailocins serve different ecological uses, such as to kill competing bacteria. One tailocin can kill one bacterial cell, and the bacteria cannot easily evolve resistance, so these agents may have applications for killing antibiotic-resistant bacteria.

Figure-1: General Structure of a Bacteriocidal Tailocin. The

structure consists of an extended sheath tube (blue), an inner rigid tail tube (orange), lipopolysaccharide (LPS)-targeting tail fibers (red), and a baseplate (gray). The tailocin attaches to a host bacterium while in its extended form (diagram right), and then contracts (diagram left) to insert the rigid spike through the cell membrane, causing membrane depolarization and cytoplasmic leakage. From: Ghequire and deMot, 2015.

The tailocin is thought to attach to a bacterium like a mature phage would, with the tail fibers attaching to specific lipopolysaccharide (LPS) residues on the host cell. The tailocin attaches in its extended form (Figure-1, right side), and (for the contractile types) the outer sheath contracts (Figure-1, left side) to expose a non-flexible inner tail tube and spike that act like a syringe to insert into through the cell membrane. The insertion depolarizes the membrane, and releases cytoplasmic contents, killing the cell. Much is known about the contractile-type T4 phage tails whose structures have been studied at 15-17 angstrom resolution (Kanamaru et al., 2002; Kostyuchenko et al., 2005). The baseplate has a hexagonal structure containing 6 tail fibers. Tail contraction is caused by a substantial rearrangement of the tail sheath proteins to cause about a one-third shortening. When inserted through the membrane, the inner tube tail extends about half the length beyond the baseplate, which is sufficient for crossing the host cell’s periplasmic space (Kostyuchenko et al., 2005). The crystalline structure of the T4 tail indicates the sheath is composed of 138 copies of the tail sheath protein, which surrounds the non-

contractile tube, and that during contraction the sheath proteins slide over each other (Aksyuk et al., 2009). Recent research indicates the T4 tail sheath resembles a stretched, coiled spring, wound around a rigid tube that has a spike-shaped protein at its tip that penetrates the bacterium

(Taylor et al., 2016). The triggering mechanism appears to be highly conserved among various bacteria and phage. The structure of the Phi-29 phage at 2.0 Angstrom resolution also shows a hexameric tube structure that forms a channel that spans the bacterial bilayer in a pore-forming mechanism similar to non-enveloped eukaryotic viruses (Xu et al., 2016).

Bacteria typically produce specialized antimicrobial compounds called bacteriocins that act upon organisms of the same or closely related species (Nakayama et al., 2000). While these bacteriocins are usually encoded on plasmid DNA, several chromosomally-encoded bacteriocins were initially discovered in Pseudomonas aeruginosa and were termed pyocins (reviewed in Shinomiya et al., 1975; Michel-Briand and Baysse, 2002). Evolutionarily, pyocin genes most likely evolved from phage genes that had inserted into the bacterial chromosome. Although they were initially discovered in P. aeruginosa, other bacteria also produce pyocins, including both gram-positive and gram-negative species (Ghequire and De Mot, 2015). The pyocins found in P. aeruginosa, however, remain the most studied and best characterized. These P. aeruginosa pyocins have since been divided into three sub-classes:

R-Type Pyocins: resemble non-flexible and contractile tails of bacteriophages, and

induce depolarization of the cytoplasmic membrane. These genes are carried in the Pseudomonas chromosome, and almost certainly evolved from integrated phage genes of the Myoviridae family.

F-Type Pyocins: also resemble phage tails, but have a flexible and non-contractile rod-

like structure. These pyocins may have evolved from Siphoviridae (non-contractile) phage genes. The killing mechanism of F-type tailocins are similar to that of R-type tailocins, however the targeting mechanism is unlike that of the R-type tailocins

(Nakayama et al., 2000).

S-Type Pyocins: colicin-like, protease-sensitive proteins, containing DNase and RNase

activity.

Bacteriocins that resemble phage tail structures, like the R and F-type pyocins of P. aeruginosa, are now being termed tailocins.

During evolution, some types of bacteria mutated (altered) the phage genes integrated within their chromosomes to suit the bacterium. Some bacteria co-opted the capsid structures, others the tail structures, and others co-opted both. The genes encoding phage tails are

especially beneficial for bacteria to co-opt because they are complex nano-machines with moving parts, so there are many functional areas to alter to suit their purposes. If the bacteria can mutate these genes to be under the control of their own secretory systems (type VI secretory system, T6SS), the tail structures are secreted outside the bacterial cell to bind to and affect other bacteria. The genes encoding tail fibers can also be mutated to bind to different species of

bacteria. The gene structure of the core components of all contractile tail-like systems appears to be highly conserved, but have diverged considerably to where ancestry can no longer be easily detected(Leiman and Shneider, 2012).

In 2013, scientists at the College of Life Sciences at Wuhan University (Wuhan, China) identified the first tailocin structure from Stenotrophomonas maltophilia, an important global opportunistic pathogen with multidrug-resistant strains (Liu et al., 2013). Electron microscopy revealed that the tailocin, termed maltocin P28, resembles a contractile but nonflexible phage tail structure. It is composed of two major proteins, 43 and 20 kDa in size, and their N-termini have been sequenced. The gene encoding P28 was identified, and is located within the S. maltophilia genome in an organization that is similar to that of the P2 phage genome and the R2 pyocin. In vitro, P28 showed bactericidal activity against 38 of 81 tested S. maltophilia strains.